The present invention relates to a valve drive device of an engine, and more specifically to a valve drive device in which a cam profile for driving a valve are changed according to the operating conditions of the engine.
As a valve drive device of an engine for automobiles of a DOHC type, various proposals have heretofore been made in which the opening and closing timing or a lift amount of a intake valve or an exhaust valve is changed according to the operating conditions of the engine. In such a valve drive device, the opening and closing timing or the lift amount of the valve is changed according to the operating conditions of the engine whereby the engine characteristics of torque or the like are controlled so as to suit to the current operating conditions.
According to one such valve drive device, a low speed cam and a high speed cam having cam profiles different from each other are provided on a single camshaft. The cam for driving the valve is switched according to the operating conditions of the engine. In this drive device, since there are two independent cams (low speed and high speed), the cam nose radiuses of the cams can differ greatly. Accordingly, the maximum lift amount of the valve can be made sufficiently small for the low speed range of the engine, and sufficiently large for the high speed range. It is therefore easy to obtain the desired engine characteristics for the current speed range.
On the other hand, for example Japanese patent Laid-open No. Hei 3-179116 Publication discloses a valve drive device in which one kind of a cam 40, the cam nose radius of which varies in the axial direction of a camshaft 42, is provided on a single camshaft 42, as shown in FIG. 7. Each cam 40 on the camshaft 42 is moved in the axial direction together with the camshaft 42 by a shaft moving mechanism 41. Each valve 43 is pressed against a cam surface 40a of the cam 40 through a cam follower 45. Two valves 43 neither intake or exhaust valves) are arranged with respect to one cylinder of the engine. Accordingly, the cams 40 are moved in the axial direction together with the camshaft 42 whereby the region of the cam surface 40a in contact with the cam follower 45 changes. As a result, the maximum lift amount of each valve 43 changes. The range of change of the maximum lift amount (hereinafter, referred to as the lift control amount) is determined according to the difference between the maximum value and the minimum value of the radius of the cam nose.
Accordingly, the lift control amount may be increased by increasing the width W in the axial direction of the cams 40 and increasing the difference between the maximum value and the minimum value of the radius of the cam noses. However, the camshaft 42 is supported by a journal bearing 44, which is between tho two cams 40. For this reason, the moving amount D in the axial direction of the cams 40 is restricted by the interference of the cams 40 and the bearing 44. Accordingly, the width W and the moving amount D of the cam 40 is restricted.
A large lift control amount may be obtained without increasing the width W and the moving amount D of the cam 40 by increasing the inclination angle .theta. of the cam surface 40a at the cam nose. By doing so, the difference between the maximum value and the minimum value of the radius of the cam nose becomes large.
Alternatively, the width S of a sliding contact surface 45a of the cam follower 45, which is in sliding contact with the cam surface 40a, may be made small. By doing so, the effective range in the axial direction of the cam surface 40a along which the cam follower 45 can be moved becomes relatively large. As a result, the difference between the maximum value and the minimum value of the radius of the cam nose can be effectively utilized. This achieves a large lift control amount without increasing the width W of the cam 40.
However, when the inclination angle e of the cam surface 40a at the cam nose is made large, the axial component of the load on the shaft 42 applied to the camshaft 42 from each valve 43 increases. Accordingly, the driving force of the shaft moving mechanism 41 must be increased, and the moving mechanism 41 becomes large accordingly. Moreover, the increase of the load in the axial direction of the shaft 42 is not preferable for the stabilized driving of the valve 43.
Further, when the width S of the sliding surface 45a of the cam follower 45 is made small, the pressure receiving area of the sliding surface 45a becomes small. Therefore, the surface pressure applied to the sliding surface 45a increases. As a result, the cam follower 45 tends to wear. Further, it is necessary to increase the moving amount D of the cam 40 in order to take advantage of the difference between the maximum value and the minimum value of the radius of the cam nose. However, since the moving amount D of the cam 40 is restricted, the lift amount is limited.